90L in the Gulf disorganized; Leslie almost stationary; Michael hits Cat 3

A remnant of Hurricane Isaac pushed southwards through Alabama on Wednesday and emerged over the Gulf of Mexico, and this disturbance is now being tracked as Invest 90L. Long-range radar out of New Orleans shows only a small area of heavy rainfall associated with 90L. The echoes show a little spiral banding behavior, and there is some slight evidence of rotation to the echoes. Visible satellite loops and surface observations from buoys and oil rigs in the Gulf suggest that 90L has formed an ill-defined, elongated surface circulation. The area covered by heavy thunderstorms is relatively small, and is pushed to the south side of the circulation center by strong northerly winds that are creating a high 20 - 30 knots of wind shear. There is a large amount of dry air that surrounds 90L on all sides that is interfering with development. A hurricane hunter aircraft is scheduled to investigate 90L this afternoon, but this flight may be cancelled if 90L does not show more organization in the next few hours.


Figure 1. Invest 90L off the coast of the Florida Panhandle as seen by NASA's Aqua satellite at 4:30 pm EDT Wednesday September 5, 2012. Image credit: NASA.

Forecast for 90L
Wind shear is predicted to fall to the moderate range, 10 - 20 knots, by tonight. Ocean temperatures in the Gulf have been cooled down considerably by the passage of Hurricane Isaac last week, and are 28.5° - 29°C. This is plenty warm enough to support formation of a tropical storm, and I expect 90L will increase in organization today and Friday as it moves slowly south-southwest. A trough of low pressure and an associated surface cold front will move southeastwards over the northern Gulf of Mexico on Sunday, and this trough should be capable of pulling 90L to the northeast to a landfall along the Florida Panhandle or west coast of Florida on Sunday. In their 8 am Tropical Weather Outlook, NHC gave 90L a 40% chance of developing into a tropical depression by Saturday morning. I put these odds a bit higher, at 50%.

Leslie remains nearly stationary
Hurricane Leslie continues to remain nearly stationary south of the island of Bermuda. Moderately high wind shear of 10 - 20 knots due to strong upper-level winds out of the west drove dry air to Leslie's west into the core of the storm last night, eroding away Leslie's eye. However, satellite loops show that Leslie is pulling a curved band of heavy thunderstorms around the west side of the center, in an attempt to form a new eye. Leslie's slow forward speed means that the storm is staying over the cold water stirred up by the storm's winds, inhibiting intensification, and NOAA buoy 41049 recorded a 1°C (1.8°F) drop in water temperature over the past 24 hours. A uncrewed Global Hawk NASA research aircraft is scheduled to fly in the stratosphere above Leslie this evening to study the hurricane's upper-level outflow. An Air Force hurricane hunter aircraft is scheduled to do a regular mission on Friday afternoon.


Figure 2. Hurricane Leslie as seen by NASA's Aqua satellite at 1:15 pm EDT Wednesday September 5, 2012. At the time, Leslie was a Category 1 hurricane with 75 mph winds. Image credit: NASA.

Forecast for Leslie
Leslie will stay stuck in a weak steering current environment until a strong trough of low pressure approaches the U.S. East Coast on Saturday. The timing of this trough is such that Leslie will be pulled northwards and then north-northeastwards over the weekend. There is significantly less agreement among the models today in the timing and speed of Leslie's track, though. The models have shifted eastwards, which lessens the threat to Bermuda and puts the island on the weak (left) side of the hurricane. If the official NHC forecast verifies, tropical storm-force winds will not begin on Bermuda until late Saturday night or early Sunday morning. According to the latest SHIPS model forecast, the shear is expected to fall to the low category, 5 - 10 knots, by Friday night. Leslie is over warm ocean waters of 29 - 30°C, and the reduction in shear and warm waters should aid intensification, and potentially allow Leslie to be at Category 2 strength at its closest pass by Bermuda Sunday morning, as indicated by the official NHC forecast. The latest 11 am EDT NHC wind probability forecast calls for a 40% chance that Leslie will be a Category 2 or stronger hurricane Sunday morning at 8 am EDT. Leslie is a huge storm, and tropical storm-force winds are expected to extend outward from its center 230 miles by Sunday.

Most of the models still indicate Leslie is likely to make landfall in Canada, but have shifted eastwards towards Newfoundland and away from Nova Scotia. The GFS model predicts a Thursday landfall in Newfoundland, but the ECMWF model is much faster, predicting a Tuesday landfall in Newfoundland. Given the wide spread in model guidance, what Leslie might do as it approaches Canada is highly uncertain. Large swells from Leslie are pounding the entire Eastern Seaboard, and these waves will increase in size as Leslie grows in size and strength this week.


Figure 3. Morning satellite image of Hurricane Michael.

Hurricane Michael hits Category 3
The first major hurricane of the 2012 Atlantic hurricane season is Hurricane Michael, which put on a unanticipated round of rapid intensification last night to become a Category 3 storm with 115 mph winds. Michael is the 7th hurricane of the season, putting 2012 in 3rd place behind 1893 and 1886 for earliest formation date of the season's 7th hurricane. We made it through 12 named storms before getting our first major hurricane, which is a rare occurrence. The only times the Atlantic has had as many as 12 named storms before getting a major hurricane was in 1936 and 1934. In both years, Hurricane 13 was the first major hurricane (note, though, that the 5th storm of 1936 is listed as a Category 3 landfall in Florida, but had maximum winds of 90 mph--definitely not Cat 3 winds--so there is a problem with the hurricane database for this storm.) Satellite loops show that Michael is an impressive storm with a well-developed eye, excellent spiral banding, and solid upper-level outflow. Michael is far out over the open Atlantic, and none of the models show that Michael will threaten any land areas during the coming seven days. Michael has likely peaked in intensity, and will not reach Category 4.

Elsewhere in the Atlantic
Most of the reliable computer models are predicting that a new tropical wave due to move off the coast of Africa on Friday will develop into a tropical depression by the middle of next week. This wave is predicted to exit Africa too far north to threaten the Lesser Antilles Islands, but it is too early to be confident of this.

I'll have a new post this afternoon.

Jeff Masters